1. Field of the Invention
The invention relates to the stabilisation and aiming of antennas, more particularly satellite telecommunication ship mounted antennas, on ships at sea subjected to accelerations and angular movements of large amplitude compared with the acceptable tolerance as regards the aiming of the antenna.
In this respect it should be remembered that the various types of antennas whose use is recommended by international telecommunication organisations have very various characteristics as regards mass and inertia on the one hand, and the required aiming accuracy on the other. In all cases the device for stabilising and aiming the antenna must take into account features specific to the antenna selected.
2. Description of the Prior Art
For a long time many solutions have been proposed for the problem of stabilising and aiming members borne by a ship. Some of these solutions, for example, those adopted for long-distance aiming devices and the guns of warships, are highly complex and require course and vertical references to be available. They cannot be transferred to merchant vessels because of their high cost and the absence of a vertical reference, since as a rule the gyrocompass of a merchant vessel supplies only a course reference.
However, in the recent past antenna-stabilising devices have been proposed which are specifically intended for maritime telecommunication by satellite. They include the one described in the Paper by M. B. Johnson entitled "Antenna control for a ship terminal for MARISAT" (IEEE Conference Publication No 160, 7-9 March 1978); this is of the kind comprising, on a base, a mounting having bearing orientational means and supporting a gyroscopic assembly with two degrees of freedom, whose outer cardan transmission has an axis of rotation (axis X) perpendicular to the bearing axis, its inner cardan transmission having an axis of rotation (axis Y) at right-angles to the axis X and being connected to the antenna during aiming.
The device disclosed in this Paper whose type is at present known as "X-Y bearing", uses for stabilisation two gyrometers mounted on the rear of the antenna, and adapted to stabilise the axes X and Y respectively. However, the device requires a vertical reference for the X axis, which is obtained by means of an accelerometer or an inclinometer mounted on the bearing axis. The voltage delivered by the accelerometer or inclinometer is subtracted from the measurement of the orientation in situ of the X Axis. The angle of elevation can be obtained only by means of a filter with a high time constant.
Clearly, these particular features mean that the device is not very satisfactory for use on merchant vessels of low tonnage, whose equipment must remain economic.
Mountings have also been proposed with four axes, comprising a platform stabilised around rolling and pitching axes by a hanging assembly and two flywheels. The aiming device is separate in that case. It is carried by the platform and enables the antenna to be orientated around the conventional azimuthal and elevational aiming axes. Clearly, such an arrangement is extremely complex. Yet another arrangements uses a triaxial mounting of the "X, Y, bearing" type, but has two flywheels each having its own cardan transmission, thus considerably increasing costs and space occupied.